Electron microscopes have been available in the past as microscopes for observing sample surfaces at a high magnification. However, since an electron beam will only travel through a vacuum, such microscopes have suffered from various problems in terms of experimental techniques. In recent years, however, a microscopic technique known as a “scanning probe microscope” has been developed which makes it possible to observe surfaces at the atomic level even in the atmosphere. In this microscope, when the probe needle at the tip end of the probe is caused to approach very close to the sample surface at an atomic size, physical and chemical actions of the individual atoms of the sample can be detected, and an image of the sample surface can be developed from detection signals while the probe needle is scanned over the surface.
The first microscope of this type is a scanning tunnel microscope (also abbreviated to “STM”). Here, when a sharp probe needle located at the tip end is caused to approach to a distance at which the attractive force from the sample surface can be sensed, e.g., approximately 1 nm (attractive force region), a tunnel current flows between the atoms of the sample and the probe needle. Since there are indentations and projections on the sample surface at the atomic level, the probe needle is scanned across the sample surface while being caused to approach and recede from the sample surface so that the tunnel current remains constant. Since the approaching and receding signals from the probe needle correspond to the indentations and projections in the sample surface, this device can pick up an image of the sample surface at the atomic level. A weak point of this device is that the tip end of the probe needle made of a conductive material must be sharpened in order to increase the resolution.
The probe needle of an STM is formed by subjecting a wire material of platinum, platinum-iridium or tungsten, etc., to a sharpening treatment. Mechanical polishing methods and electrolytic polishing methods are used for this sharpening treatment. For example, in the case of platinum-iridium, a sharp sectional surface can be obtained merely by cutting the wire material with the nippers of a tool. However, not only is the reproducibility inaccurate, but the curvature radius of the tip end is large, i.e., around 100 nm, and such a curvature radius is inadequate for obtaining sharp atomic images of a sample surface with indentations and projections.
Electrolytic polishing is utilized for tungsten probe needles. FIG. 25 is a schematic diagram of an electrolytic polishing apparatus. A platinum electrode 80 and a tungsten electrode 81, which constitutes the probe needle, are connected to an AC power supply 82 and are suspended in an aqueous solution of sodium nitrite 83. As current flows, the tungsten electrode 81 is gradually electrolyzed in the solution, so that the tip end of this electrode is finished into the form of a needle. When polishing is completed, the tip end is removed from the liquid surface; as a result, a tungsten probe needle 84 of the type shown in FIG. 26 is completed. However, even in the case of this tungsten probe needle, the curvature radius of the tip end is about 100 nm, and indentations and projects formed by a few atoms or more cannot be sharply imaged.
The next-developed scanning type probe microscope is the atomic force microscope (abbreviated as “AFM”). In the case of an STM, the probe needle and sample must as a rule be conductors in order to cause the flow of the tunnel current. Accordingly, the AFM is to observe the surfaces of non-conductive substances. In the case of this device, a cantilever 85 of the type shown in FIG. 27 is used. The rear end of this cantilever 85 is fastened to a substrate 86, and a pyramid-form probe needle 87 is formed on the front end of the cantilever 85. A point part 88 is formed on the tip end of the probe needle by a sharpening treatment. The substrate 86 is mounted on a scanning driving part. When the point part is caused to approach the sample surface to a distance of approximately 0.3 nm from the sample surface, the point part receives a repulsive force from the atoms of the sample. When the probe needle is scanned along the sample surface in this state, the probe needle 87 is caused to move upward and downward by the above-described repulsive force in accordance with the indentations and projections of the surface. The cantilever 85 then bends in response to this in the manner of a “lever”. This bending is detected by the deviation in the angle of reflection of a laser beam directed onto the back surface of the cantilever, so that an image of the surface is developed.
FIG. 28 is a diagram of the process used to manufacture the above-described probe needle by means of a semiconductor planar technique. An oxide film 90 is formed on both surfaces of a silicon wafer 89, and a recess 91 is formed in one portion of this assembly by lithography and etching. This portion is also covered by an oxide film 92. The oxide films 90 and 92 are converted into Si3N4 films 93 by a nitrogen treatment; then, the entire undersurface and a portion of the upper surface are etched so that a cut part 94 is formed. Meanwhile, a large recess 96 is formed in a glass 95, and this is anodically joined to the surface of the Si3N4 film. Afterward, the glass part 97 is cut, and the silicon part 98 is removed by etching. Then, the desired probe needle is finished by forming a metal film 99 used for laser reflection. Specifically, the cantilever 85, substrate 86, probe needle 87 and point part 88 are completed.
This planar technique is suited for mass production; however, the extent to which the point part 88 can be sharpened is a problem. In the final analysis, it is necessary either to sharply etch the tip end of the recess 91, or to sharpen the tip end of the probe needle 87 by etching. However, even in the case of such etching treatments, it has been difficult to reduce the curvature radius of the tip end of the point part 88 to a value smaller than 10 nm. The indentations and projections on the sample surface are at the atomic level, and it is necessary to reduce the curvature radius of the tip end of the point part 88 to a value of 10 nm or less in order to obtain sharp images of these indentations and projections. However, it has been impossible to achieve such a reduction in the curvature radius using this technique.
If artificial polishing and planar techniques are useless, the question of what to use for the probe needle, which is the deciding element of the probe, becomes an important problem. One approach is the use of whiskers (whisker crystals). Zinc oxide whiskers have actually been utilized as probe needles. Whisker probe needles have a smaller tip end angle and tip end curvature than pyramid needles produced by planar techniques, and therefore produce sharper images. However, whisker manufacturing methods have not been established, and the manufacture of conductive whiskers for STM use has not yet been tried. Furthermore, whiskers with the desired cross-sectional diameter of 10 nm or less have not yet been obtained.
Furthermore, such probe needles have suffered from many other problems: e.g., such probe needles are easily destroyed by strong contact with the sample surface, and such needles quickly become worn under ordinary use conditions, so that use becomes impossible.
In recent years, therefore, the idea of using carbon nanotubes as probe needles has appeared. Since carbon nanotubes are conductive, they can be used in both AFM and STM. A carbon nanotube probe needle has been proposed as a high-resolution probe for imaging biological systems in J. Am. Chem. Soc., Vol. 120 (1998), p. 603. However, the most important points, i.e., the question of how to collect only carbon nanotubes from a carbon mixture, and the question of how to fasten carbon nanotubes to a holder, remain completely unsolved. In this reference as well, the use of an assembly in which a carbon nanotube is attached to a holder by means of inter-molecular force is mentioned only in passing.
Furthermore, besides carbon nanotubes, BCN type nanotubes and BN type nanotubes have also been developed as nanotubes. However methods of utilizing such nanotubes have remained completely in the realm of the unknown.
On a different subject, memory devices have evolved from floppy disk drives to hard disk drives, and further to high-density disk drives, as the memory capacity of computers has increased in recent years. As information is packed into smaller spaces at higher densities, the size per bit of information decreases; accordingly, a finer probe needle is also required for input-output. In conventional magnet head devices, it is impossible to reduce the size of the probe needle beyond a certain fixed value, so that there are limits to the trend toward higher density.
As described above, systematic conventional techniques for sharpening probe needles are electrolytic polishing of metal wire materials and lithography and etching treatments of semiconductors. In the case of these treatments, however, the tip end curvature radius of the probe needle can only be sharpened to about 100 nm; accordingly, it is very difficult to obtain sharp images of indentations and projections formed by a few atoms or more on the sample surface. Furthermore, the degree of sharpness obtained by mechanically cutting metal wire materials with a tool such as nippers, etc. is also insufficient to capture sharp images of indentations and projections. The use of whiskers is still an uncertain technique, and the use of nanotube probe needles such as carbon nanotubes, etc. has been a task for the future. Furthermore, conventional magnetic head devices have also approached their limit in terms of size.
Accordingly, the object of the present invention is to provide the utilization of nanotubes with a small tip end curvature radius as surface signal operating probe needles and further to establish a concrete structure for probes using nanotube probe needles, and a method for manufacturing the same. The present invention shows that such nanotube probe needles are not easily destroyed even when they contact atomic-level projections during probe needle scanning, that such probe needles can be firmly fastened to the holder so that the probe needle will not come loose from the holder during such scanning, and that such probe needles can be inexpensively mass-produced. Furthermore, the present invention shows that samples that could not be observed with high resolution in the past can be clearly observed using the nanotube probe needles thus manufactured.